MM1: Basic Concept (I): System and its Variableshomes.et.aau.dk/yang/DE5/CC/summary.pdf9/9/2011...

9/9/2011 Classical Control 1

MM1: Basic Concept (I): System and its Variables

A system is a collection of components which are coordinated together to perform a function

Systems interact with their environment. The interaction is defined in terms of variables System inputs System outputs Environmental disturbances

Dynamic system is a system ehose performance could change according to time


MM1: Basic Concept (II): Control Control is a process of causing a system (output) variable to

conform to some desired status/value Manual Control is a process where the control is handled by

human being(s) Automatic Control is a control process which involves

machines only


MM1: Control Classification Open-loop Control: A control process which does not utilize the

feedback mechanism, i.e., the output(s) has no effect upon the control input(s)

Closed-loop Control: A control process which utilizes the feedback mechanism, i.e., the output(s) does have effect upon the control input(s)

Reference/Set-point


MM1: Feedback Control – Block Diagrams

Forwardcompensator actuator Plant

sensorFeedback

compensator

+

-

Reference input

disturbance

D(s) A(s) P(s)

S(s)F(s)

+

-r

w

The Goals of this lecture (MM2) ... Essentials in using (ordinary) differential equation model

Why use ODE model Linear vs. nonlinear ODE models How to solve an ODE Numerical methods (Matlab)

Refresh of Laplace transform Key features Transformation from ODE to TF model

Block diagram transformation Composition /decomposition Signal-flow graph


MM2: ODE Model A general ODE model:

SISO, SIMO, MISO, MIMO models Linear system, Time-invance, Linear Time-Invarance (LTI) Solution of ODE is an explicit description of dynamic behavior Conditions for unique solution of an ODE Solving an ODE:

Time-domain method, e.g., using exponential function Complex-domain method (Laplace transform) Numerical solution – CAD methods, e.g., ode23/ode45


MM2: Block diagram Rules


MM2: Simulink Block diagram System build-up

Using TF block Using nonlinear blocks Using math blocks

Creat subsystems Top-down Bottom-up

Usage of ode23 & ode45



Goals for this lecture (MM3) Time response analysis Typical inputs 1st, 2nd and higher order systems

Performance specification of time response Transient performance Steady-state performance

Numerical simulation of time response

9/9/2011 Analog and Digital Control 10

MM3: Time Response Analysis (I)

d(t)=0

Typical input u(t)

Time response y(t)

TF model

)0(,)0( mm

),,),0(),0(( feammm TTvf

Laplace Trans Inv Laplace T.

n

i

ii

m

i

ii

sa

sbsG

sUsGsY

0

0)(

)()()(

MM3: Time Response Analysis (II)

Typical inputs: impulse, step and ramp signals

1st, 2nd and high-order (LTI) systems


tpt ectgketg

scsG

pp

psksG

1

)(or )(

:domain time

:constant time,1:pole,1

)(

1 :constant time,:pole,)(

Time response = excitation response + initial condition response(free response)


21,

3.0%,355.0%,16

7.0%,5

6.46.4

8.1

ndd

p

p

ns

nr

t

M

t

t

MM3: Performance Specification

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MM3: Numerical Simulation Impulse response: impulse(sys) Step response: step(sys) ltiview(sys) Subplot(m,n,1)EXAMPLE:sys1: Sys2: num1=[1]; num2=[1 2];den1=[1 2 1]; den2=[1 2 3];impulse(tf(num1,den1),'r',tf(num2,den2),'b')step(tf(num1,den1),'r',tf(num2,den2),'b')


Goals for this lecture (MM4) System poles vs. time responses Poles and zeros Time responses vs. Pole locations

Feedback characteristics Characteristics A simple feedback design

Block diagram decomposition (simulink)

MM4 : Poles vs Performance


Pole locations Time response

MM4: First-order System

9/9/2011 Classical Control

16

t

t

e)ss

L(y(t)

e)s

L(y(t)

ssG

1

1

1)1(

1 :response Step

11 :response Impulse

, :constant time,1:pole

0 assume,1

1)(

Time constant – why?

63%

Time response is determined by the time constantSystem pole is the negative of inverse time constant


MM4: Second-Order System

0 if , :polescomplex

10 if ,1 :polescomplex

1 if , :poles )(identical real

1 if,1 :poles )(different real

1:poles

0,0 assume,2

)(

2,1

22,1

2,1

22,1

22,1

22

2

n

nn

n

nn

nn

n

nn

n

jp

jp

ξp

p

p

ssG

21,

3.0%,355.0%,16

7.0%,5

6.46.4

8.1

ndd

p

p

ns

nr

t

M

t

t


MM4: Summary of Pole vs Performance


MM4: Plot of Pole Locations

s1=tf(1,[1 2 1]); s2=tf(1,[1 1.6 1]);s3=tf(1,[1 1.0 1]);s4=tf(1,[1 0 1]);pzmap(s1,s2,s3,s4)sgrid


Goals for this lecture (MM5)

Stability analysis Definition of BIBO Pole locations Routh criteron

Steady-state errors Final Theorem DC-Gain Stead-state errors

Effects of zeros and additional poles

MM5 : BIBO Stability A system is said to have bounded input-bounded output

(BIBO) stability if every bounded input results in a bounded output (regardless of what goes on inside the system)

The continuous (LTI) system with impuse response h(t) is BIBO stable if and only if h(t) is absolutely integrallable

All system poles locate in the left half s-plane - asymptotic internal stability

Routh Criterion: For a stable system, there is no changes in sign and no zeros in the first column of the Routh array


MM5 : Steady-State Error Objective: to know whether or not the response of a system

can approach to the reference signal as time increases Assumption: The considered system is stable Analysis method: Transfer function + final-value Theorem

Position-error constant Velocity constant Acceleration constant


)0(1))(1(lim

1)(),())(1(lim

))()()((lim))()((lim)(

0

0

00

GsGs

sRsRsGs

sRsGsRssYsRse

s

s

ss

DC-Gain

)(lim

)(lim

)(lim

2

0

0

0

sGsK

ssGK

sGK

osa

osv

osp

MM5 : Effect of Additional Zero & PoleC

hapt

er 6

9/9/2011 23Classical Control

An additional zero in the left half-plane will increase the overshootIf the zero is within a factor of 4 of the real part of the complex poles

An additional zero in the right half-plane will depress the overshootand may cause the step response to start out in the wrong direction

An additional pole in the left half-plane will increase the rise time significantly if the extra pole is within a factor of 4 of the real part of the complex poles

BIBO Stability – Execise (I)

Are these systems BIBO stable?

Intuitive explanation Theoretical analysis


1

2

3

4

BIBO Stability – Execise (II) How about the stability of your project systems?


Revisit of example: First-order System (II)


9/0.9510/9

What’s the tpye of original system? Derive the transfer function of the closed-loop system What’s the time constant and DC-gain of the CL system? What’s the feedforward gain so that there is no steady-state

error?


Goals for this lecture (MM6) Definition characterisitc of PID control

P- controller PI- controller PID controller

Ziegler-Nichols tuning methods Quarter decay ratio method Ultimate sensitivity method

Control objectivesControl is a process of causing a system (output) variable to

conform to some desired status/value (MM1)


Reference/Set-point

Control Objectives Stable (MM5) Quick responding (MM3, 4) Adequate disturbance rejection Insensitive to model &

measurement errors Avoids excessive control action Suitable for a wide range of

operating conditions(extra readings: Goodwin’s lecture)

MM6:Characteristics of PID Controllers Proportional gain, Kp larger values typically mean faster

response. An excessively large proportional gain will lead to process instability and oscillation.

Integral gain, Ki larger values imply steady state errors are eliminated more quickly. The trade-off is larger overshoot

Derivative gain, Kd larger values decrease overshoot, but slows down transient response and may lead to instability due to signal noise amplification in the differentiation of the error.


K(1+1/Tis+ TDs)PlantG(s)

+

-R(s) E(s) Y(s)

MM6: PID Tuning Methods- Trial-Error


See Hou Ming’s lexture notes

MM6: PID Tuning – Zieglor Niechols (I)


Pre-condition: system has no overshoot of step response


MM6: PID Tuning – Zieglor Niechols (II)


Pre-condition: system order > 2



Goals for this lecture (MM7)Some practical issues when developing a PID controler:

Integral windup & Anti-windup methods Derivertive kick When to use which controller? Operational Amplifier Implementation Other tuning methods


Anti-windup Techniques

Derivative Kick Derivative kick: if we have a setpoint change, a

spike will be caused by D controller, which is called derivative kick.

Derivative kick can be removed by replacing the derivative term with just output (y), instead of (rset-y)

Derivative kick can be reduced by introducing a lowpass filter before the set-point enters the system

The bandwidth of the filter should be much larger than the closed-loop system’s bandwidth


)()()11(

))(1)(()( 0

ssYTsEsT

KU(s)

(t)yTdeT

teKtu

DI

D

t

tI

Cohen-Coon Tuning Method Pre-condition: first-order system with some time delay Objective: ¼ decay ratio & minimum offset


( ) (1st order)1

sKeG ss

9/9/2011 37Classical Control

( ) (1st order)1

sKeG ss


Goals for this lecture (MM8)Essentials for frequency domain design methods – Bode plot

Bode plot analysis How to get a Bode plot What we can gain from Bode plot

How to use bode plot for design purpose Stability margins (Gain margin and phase margin) Transient performance Steady-state performance

Matlab functions: bode(), margin()


Frequency Response

The frequency response G(j) (=G(s)|s=j) is a representation of the system's response to sinusoidal inputs at varying frequencies

G(j) = |G(j)| eG(j), Input x(n) and output y(n) relationship

|Y(j)| = |H(j)| |X(j)|Y(j) = H(j) + X(j)

The frequency response of a system can be viewed via the Bode plot (H.W. Bode 1932-1942) via the Nyquist diagram

G(s)X(s) Y(s)


Open-Loop Transfer Function Motivation

Predict the closed-loop system’s properties using the open-loop system’s frequency response

Open-loop TF (Loop gain) :

Closed-loop:

G(s)KD(s)

L(s)=KD(s)G(s)

G(s)

KD(s)

Gcl(s)=L(s)/(1+L(s)), or Gcl(s)=G(s)/(1+L(s))


Definition of Phase Margin (PM) Bode plot of the open-

loop TF

The phase margin is the difference in phase between the phase curve and -180 deg at the point corresponding to the frequency that gives us a gain of 0dB (the gain cross over frequency, Wgc).


Remarks of Using Bode Plot Precondition: The system must be stable in open loop if we

are going to design via Bode plots Stability: If the gain crossover frequency is less than the

phase crossover frequency (i.e. Wgc < Wpc), then the closed-loop system will be stable

Damping Ratio: For second-order systems, the closed-loop damping ratio is approximately equal to the phase margin divided by 100 if the phase margin is between 0 and 60 deg

A very rough estimate that you can use is that the bandwidth is approximately equal to the natural frequency


Goals for this lecture (MM9) A design example based on Bode plot

Open-loop system feature analysis Bode plot based design

Nyquist Diagram What’s Nyquist diagram? What we can gain from Nyquist diagram

Matlab functions: nyquist()


Nyquist Diagram: DefinitionThe Nyquist diagram is a plot of G(j) , where G(s) is the open-loop transfer function and is a vector of frequencies which encloses the entire right-half plane

G(j) = |G(j)| eG(j), The Nyquist diagram plots the position its the complex

plane , while the Bode plot plots its magnitude and phase separately.


Nyquist Criterion for Stability (MM9)The Nyquist criterion states that: P = the number of open-loop (unstable) poles of G(s)H(s) N = the number of times the Nyquist diagram encircles –1

clockwise encirclements of -1 count as positive encirclements

counter-clockwise (or anti-clockwise) encirclements of -1 count as negative encirclements

Z = the number of right half-plane (positive, real) poles of the closed-loop system

The important equation: Z = P + N


Goals for this lecture (MM10) An illustrative example

Frequency response analysis Frequency response design

Lead and lag compensators What’s a lead/lag compensator? Their frequency features

A systematical procedure for lead compensator design

A practical design example – Beam and Ball Control


What have we talked in lecture (MM10)? Lead and lag compensators

D(s)=(s+z)/(s+p) with z < p or z > p

D(s)=K(Ts+1)/(Ts+1), with <1 or >1

A systematical procedure for lead compensator design

max

max

max

sin1sin1

1

T Controller

KD(s)PlantG(s)


Exercise Could you repeat the antenna design using

1. Continuous lead compensation; 2. Emulation method for digital control;

Such that the design specifications: Overshoot to a step input less than 5%; Settling time to 1% to be less than 14 sec.; Tracking error to a ramp input of slope 0.01rad/sec to

be less than 0.01rad; Sampling time to give at at least 10 samples in a rise

time. (Write your analysis and program on a paper!)


1. Introduction - Root Locus

• The root locus of an (open-loop) transfer function KG(s) is a plot of the locations (locus) of all possible closed loop poles with proportional gain K and unity feedback

• From the root locus we can select a gain such that our closed-loop system will perform the way we want

G(s)K

Open-loop trans. Func.: KG(s); Closed-loop trans. Func.: KG(s)/(1+KG(s))Sensitivity function: 1/(1+KG(s))


Control Design Using Root Locus (I) Objective: select a

particular value of K that will meet the specifications for static and dynamic

1+KG(s)=0

Magnitude condition:K=1/|G(s)|


Exercise Question 5.2 on FC page.321; Consider a DC motor control using a PI controler

Where the motor is modeled as G(s)=K/(s+1) and PI controller is D(s)=Kp(Tis+1)/Tis, with parameters K=30, =0.35, Ti=0.041. Through the root locus method determine the largest vaule of Kp such that =0.45

Try to use the root locus method to design a lead compensator for the examplifed attenna system.

G(s)D(s)

MM1: Basic Concept (I): System and its Variableshomes.et.aau.dk/yang/DE5/CC/summary.pdf9/9/2011...

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